The software giant’s allegations are the third high-profile assault on Google’s Android platform this year, following lawsuits by Apple against HTC and Oracle against Google.

Android, which Google allows phone manufacturers to use for free, has surged in popularity in less than two years — recently passing AT&T’s iPhone in market share thanks to its presence on all four major U.S. wireless carriers.

“It just seems to me that everyone wants to stop the big Google freight train,” said Will Stofega, a mobile-industry analyst at IDC.

“These kinds of litigations aren’t cheap,” he added. “And the fact they’re going after it and spending all this money shows how high the stakes are.”

Download Microsoft’s complaint to the International Trade Commission (PDF).

Download Microsoft’s complaint filed in U.S. District Court for the Western District of Washington (PDF).

Horacio Gutierrez, Microsoft’s corporate vice president and deputy general counsel of Intellectual Property and Licensing, explained today’s legal actions in the following statement:

Microsoft filed an action today in the International Trade Commission and in the U.S. District Court for the Western District of Washington against Motorola Inc. for infringement of nine Microsoft patents by Motorola’s Android-based smartphones. The patents at issue relate to a range of functionality embodied in Motorola’s Android smartphone devices that are essential to the smartphone user experience, including synchronizing email, calendars and contacts, scheduling meetings, and notifying applications of changes in signal strength and battery power.

We have a responsibility to our customers, partners, and shareholders to safeguard the billions of dollars we invest each year in bringing innovative software products and services to market. Motorola needs to stop its infringement of our patented inventions in its Android smartphones.

Microsoft said Motorola licensed some of its patented mobile technology from 2003 to 2007, but continued to use it after the license expired. Such agreements are common — HTC and Microsoft signed a licensing deal in April over its Android phones — in this day of interconnected mobile devices.

“Everybody is being forced to use so many other people’s technology,” said Henry Sneath, an intellectual property attorney at Picadio Sneath Miller & Norton in Pittsburgh. “To compete, to give consumers all the features they want, you’re going to have to license, copy, imitate or steal someone else’s technology and put it in your phone.”

“It’s kind of inevitable,” he said of such lawsuits. “I’m surprised it hasn’t happened more.”

Microsoft’s allegations are essentially a proxy attack on Google. And the Redmond-based company is not alone in its endeavor.

In March, Apple filed complaints against HTC and its Android-based phones, alleging the company violated 20 of its patents. And in August, Oracle sued Google itself over the use of a Java-like platform in Android.

HTC, which is the other big Android user, seems to be safe from Microsoft’s legal team thanks to the companies’ intellectual property agreement over Android. Microsoft didn’t say what specific Microsoft technology HTC uses in Android, but odds are they are similar to what Microsoft is saying about Motorola’s Android phones.

Jennifer Erickson, Motorola’s chief communications officer, responded to the allegations with the following statement:

Motorola has not received a copy of the complaint, therefore we cannot comment at this point. Motorola has a leading intellectual property portfolio, one of the strongest in the industry. The company will vigorously defend itself in this matter.

As Microsoft gears up to launch Windows Phone 7, it is hoping to get as many wireless carriers and phone manufactures on board as possible. Motorola, with its successful Droid line on Verizon, has been strengthening ties with Google and its Android operating system.

Meanwhile, in the past month, the Windows Phone 7 launch has lost momentum. Microsoft delayed the platform’s support of CDMA-protocol wireless networks such as Verizon and Sprint, admitting this fall’s version of Windows Phone 7 will support only the GSM wireless protocol.

And recent reports put an AT&T-only launch on Nov. 8, pitting the new OS in a head-to-head fight against the wildly popular AT&T-only iPhone. As such, Microsoft will miss out on the opportunity to capitalize on AT&T customers’ desires to leave what is largely viewed as the wireless network with the worst service — a consumer attitude that has helped Android considerably.

Who knows how much more deeply ingrained Android will get in the next few months, before Microsoft is able to get Windows Phone 7 devices out on Verizon, Sprint and T-Mobile.

The recent slew of Android-targeted infringement lawsuits seems to have one purpose, Stofega said: “to inject a little bit of fear in the Android community, maybe a little uncertainty.”